How to find what version of Java i have ?

Java is the World mostly widely used programming environment, Everything is Java now,Then Internet is not an exceptional, For security reasons or more services, the Java runtime for browser is been required, but how you know what version of Java you have?

Some of the website that needs Java essentially, where we have to install Java to use service from a respective servers, since if it does not exist means, it need to install Java on your system, if already you have Java means, you have to update yourself for bug fixes and vulnerabilities issues,

Also some of the website or application particularly requires particular version in Java, because their virtual machine or Java runtime requires particular Java version, So you have to know what Java version you are using,So let we move know how to test what Java version we using ?